The present invention relates to fuser apparatus for electrostatographic printing machines and in particular to oversized (i.e. wide rolls) roll fusers.
In imaging systems commonly used today, a charge retentive surface is typically charged to a uniform potential and thereafter exposed to a light source to thereby selectively discharge the charge retentive surface to form a latent electrostatic image thereon. The image may comprise either the discharged portions or the charged portions of the charge retentive surface. The light source may comprise any well known device such as a light lens scanning system or a laser beam. Subsequently, the electrostatic latent image on the charge retentive surface is rendered visible by developing the image with developer powder referred to in the art as toner. The most common development systems employ developer which comprises both charged carrier particles and charged toner particles which triboelectrically adhere to the carrier particles. During development, the toner particles are attracted from the carrier particles by the charged pattern of the image areas of the charge retentive surface to form a powder image thereon. This toner image may be subsequently transferred to a support surface such as plain paper to which it may be permanently affixed by heating or by the application of pressure or a combination of both.
In order to fix or fuse the toner material onto a support member permanently by heat, it is necessary to elevate the temperature of the toner material to a point at which constituents of the toner material coalesce and become tacky. This action causes the toner to flow to some extent onto the fibers or pores of the support members or otherwise upon the surfaces thereof. Thereafter, as the toner material cools, solidification of the toner material occurs causing the toner material to be bonded firmly to the support member.
One approach to thermal fusing of toner material images onto the supporting substrate has been to pass the substrate with the unfused toner images thereon between a pair of opposed roller members at least one of which is internally heated. During operation of a fusing system of this type, the support member to which the toner images are electrostatically adhered is moved through the nip formed between the rolls with the toner image contacting the heated fuser roll to thereby effect heating of the toner images within the nip. Typical of such fusing devices are two roll systems wherein the fusing roll is coated with an abhesive material, such as a silicone rubber or other low surface energy elastomer or, for example, tetrafluoroethylene resin sold by E.I. DuPont De Nemours under the trademark Teflon. In these fusing systems, however, since the toner image is tackified by heat it frequently happens that a part of the image carried on the supporting substrate will be retrained by the heated fuser roller and not penetrate into the substrate surface. The tackified toner may stick to the surface of the fuser roll and offset to a subsequent sheet of support substrate or offset to the pressure roll when there is no sheet passing through a fuser nip resulting in contamination of the pressure roll with subsequent offset of toner from the pressure roll to the image substrate. In order to prevent this from happening, a release agent application mechanism is utilized.
Wide, small diameter roll fusers inherently suffer from excessive fuser and pressure roll deflection. The load fusers require is a function of speed and type of image to be fused. Color fusers need roughly three times the load a monochrome fuser requires, for a given speed. Bending of a beam, or roller, is inversely proportional to the cube of the length thus, as fuser get wider the rolls bend appreciably more at a given load. Likewise, the bending of a beam with a round cross section, or roller, is directly proportional to the cube of the roll radius. So if it is desired to make the roll a little smaller the deflection increases significantly. The goal in a fuser nip is to produce nearly uniform load across the width. As the roll deflects the load at the ends increase thereby producing paper handing problems, if the load is too nonuniform.
It is known that skewing the fuser roll with respect to the pressure roll will tend to counteract the uneven load distribution caused by roll bending. This occurs because of the wrapping of one roll around the other. However, the resultant shape of the roll is a curve which is a cubic function and it is being wrapped around a circular roll with is a squared function. The resulting load distribution is a maximum about one quarter of the roll length in from each end. You get a "bow tie" nip. Skewing has been successfully employed for fairly stiff systems and very flexible systems. The former needs very little compensation and thus little "bow tie" effect is apparent while the latter requires a lot of skew but the stiffness is low enough that the "bow tie" effect is not visible. Skewing also generates lateral thrust forces that wear the roll surface.
Uneven roll load distribution can also be prevented by crowning one of the two fuser rolls. However, crowning of one of two fuser rolls results in nip velocity problems which induce paper wrinkle.